Membrane separation processes are becoming increasingly popular for the separation of a feed stream into two streams, the retentate stream which is rich in a component which does not selectively pass through the membrane and a permeate stream which is rich in components which selectively pass through the membrane. Examples of such processes include gas separations, such as the separation of oxygen from nitrogen, reverse osmosis processes in which dissolved salts are removed from water, processes for converting surface water to potable water by removing particulates and high molecular weight species such as bacteria, and pervaporation processes in which volatile components of a liquid stream selectively permeate through a membrane.
In order for a membrane separation to be effective there must be some driving force. The two most common driving forces are based on a concentration or a pressure differential from one side of the membrane to the other. When pressure is a driving force the pressure is higher on the feed side of the membrane. When concentration is the driving force, the concentration of the species which preferentially permeates is typically higher on the feed side.
Recently, some plate and frame membrane separation devices have been disclosed (Japan Kokai 41-10090 (application no. 2,229,631) published 10 Apr. 1992; Japan Kokai 47023 (application no. 2,107,991) published 10 Jan. 1993 and Japan Kokai 4187222 (application no. 2,319,141) published 3 Jul. 1992) which have suction pumps on the permeate side of a plate and frame membrane. The pressure differential can be created by the suction pumps. In such devices each membrane element is a plate with a membrane attached to it wherein each plate has a permeate flow channel on the permeate side of the membrane. The suction pump is connected individually to each plate. The membrane plates are carefully separated from one another to allow turbulent flow along the surface of each membrane to facilitate removal of solids which tend to stick to the retentate side of the membrane. The plates are generally placed vertically in a tank. The turbulent flow is created by pumps, blades or air blown through aeration pipes placed near the membrane services.
U.S. Pat. No. 5,084,220 discloses apparatus including membrane cassette which is useful in converting large amounts of surface water into potable water. The disclosed apparatus comprises an array of cassette frames separated by intermediate plates. The cassette frames and intermediate plates are provided with high pressure seals so that when the cassette frames and intermediate plates fit together a pressure tight seal is formed. The whole array of cassette frames and intermediate plates form a pressure vessel when end plates which also seal with the end cassette frames are used. The array of cassette frames intermediate plates and end plates is adapted to be clamped together in its longitudinal direction and loosened, as required, to remove any selected cassette frame. Each cassette frame contains a filtration unit comprising a stack of membranes, in which a first flow passage system is provided which connects two free zones within the cassette frame, said free zones being so connected as to establish a series, parallel or combined series and parallel pattern of fluid flow from the feed inlet to a concentrate outlet. In each stack of membranes is a second system of flow passages which is isolated from the first system and serves to conduct the stream that permeates through the membranes to at least one outlet from each filtration unit. This system is designed to operate at relatively high pressures. The system has been carefully designed to allow the end plates cassette units and intermediate plates to fit together in a fashion such that a pressure case is formed about the membranes. The system also is designed to include relatively sophisticated pressure control and flow control means, and the materials used for the system have been carefully chosen to withstand such high pressures. The use of such costly materials and sophisticated process controls results in a system which may not be cost effective for low pressure applications. It is desirable to develop a system which is capable of performing low transmembrane pressure separations which does not require such sophisticated materials and process controls.
Membrane separation systems are relatively compact when compared to other known separation systems. Yet, many known membrane systems still occupy a considerable amount of space which is not directly used for performing the separation. Pressurized systems typically require external pumps, large pressure vessels, feed, permeate, concentrate and recirculation conduits and external controls, which occupy space not directly used for the separation. A system which more efficiently uses space is desirable.
A common design for membrane apparatus useful for separating liquids is the spiral wound device, which comprises a plurality of pairs of flat sheets with a spacer between the pairs wherein the pairs are sealed. These flat sheets are then rolled up into a jelly roll configuration and placed in a pressure case. An advantage of the spiral wound device is that it allows a large membrane area to be placed in a membrane module. A significant problem of such a device is that the flux is unevenly distributed over the membrane surface. One reason for this is that the pressure drop from one end of the membrane to the other end of the membrane is significant. In order to compensate for the pressure drop problem, most spiral wound membranes are operated with a large feedside pressure. A significant problem created by the use of high feedside pressures and transmembrane pressure differentials is fouling of the membrane, which results from materials which do not pass through the membrane forming a layer on the feedside of the membrane and restricting the access of the feed stream to the feedside of the membrane.
What is needed is a membrane separation system and apparatus which is relatively compact in that the space taken up by portions of the apparatus or system not directly used for separation is minimized. What is further needed is a system which provides for relatively even distribution of the feed along the entire membrane surface and which can perform the desired separation at relatively low feed pressures and transmembrane pressure differentials. Such system would thereby reduce the inherent fouling of conventional membrane separation systems. What is also needed is a system for membrane separations which does not require costly pressure vessels and controls necessary to operate in higher pressure environments.
Those systems which operate at relatively high feed pressures and transmembrane differentials also require high energy consumption in order to achieve the desired separation. What is desired is a system which can operate at low pressures thereby resulting in a considerable energy savings to perform the separation.